1. Field of the Invention
The present invention relates to a heat-conductive silicone composition which, even when filled with a large quantity of a heat-conductive filler in order to provide superior thermal conductivity, shows a minimal increase in viscosity or plasticity, and still exhibits favorable handling properties and moldability. The invention also relates to a heat-conductive silicone cured product which, even when filled with a large quantity of a heat-conductive filler, exhibits excellent flexibility.
2. Description of the Prior Art
Many electronic components generate heat during use, and in order to ensure that those electronic components function satisfactorily, heat must be conducted away from the electronic components. Particularly in the case of integrated circuit elements such as the CPUs used in personal computers, increases in the operating frequency have lead to increased heat generation, and dealing with this heat has become a significant problem.
Many methods have been proposed for removing this heat. Particularly in the case of electronic components that generate a large quantity of heat, methods have been proposed in which the heat is dissipated by placing a heat-conductive material such as a heat-conductive sheet or heat-conductive grease between the electronic component and another member such as a heat sink (see patent reference 1 and patent reference 2).
Furthermore, patent reference 3 discloses a composition in which from 60 to 500 parts by weight of silica and a heat-conductive powder of silver, gold or silicon or the like are added to an addition-curable silicone rubber, yielding a heat-radiating material that can be used in locations where insulation is not required.
However, these heat-conductive materials all have thermal conductivity values lower than 1.5 W/mK, and if a large quantity of a heat-conductive filler is added in order to improve the thermal conductivity, then either the fluidity deteriorates in the case of a liquid silicone composition, or the plasticity increases in the case of a millable silicone composition. Either result causes a dramatic deterioration in the handling properties and moldability of the composition, and cured products formed from the compositions also suffer from increased hardness and brittleness.
In an attempt to resolve this problem, patent reference 4 discloses highly heat-conductive rubber and plastic compositions filled with an alumina filler consisting of 10 to 30% by weight of alumina particles with an average particle size of not more than 5 μm, and the remainder of spherical corundum particles with no cutting edges and with an average particle size for a single particle of at least 10 μm. Furthermore, patent reference 5 discloses a heat-conductive silicone rubber composition consisting of a base formed from a combination of a gum-type organopolysiloxane with an average polymerization degree of 6,000 to 12,000 and an oily organopolysiloxane with an average polymerization degree of 200 to 2,000, and a spherical aluminum oxide powder, which is added in a ratio of 500 to 1,200 parts by weight of the aluminum oxide powder per 100 parts by weight of the base polymer components.
However even using these methods, if a high degree of filling of at least 70 volume % of aluminum oxide is used, then there is a limit to the improvement in moldability that can be achieved by simply adjusting the particle combinations or altering the viscosity of the silicone base.
Accordingly, techniques have been developed in which the surface of the inorganic filler that acts as the heat-conductive filler is treated with a surface treatment agent such as a silane coupling agent, thereby improving the dispersibility of the filler within the silicone base polymer, and enabling a higher degree of filling. For example, patent reference 6 discloses a heat-conductive silicone rubber composition containing an inorganic filler that has been surface-treated with a silane coupling agent comprising an aliphatic long-chain alkyl group and a methoxy or ethoxy group, and also discloses a heat-radiating sheet produced using the heat-conductive silicone rubber composition. However, because these types of silane-based treatment agents have comparatively low molecular weights they are prone to volatilization, meaning the viscosity or plasticity of the obtained silicone compositions tends to increase over time, and the cured products produced from the compositions suffer from increased hardness over time. If the surface treatment is conducted using the type of alkoxy group-containing organopolysiloxane disclosed in patent reference 7 and patent reference 8, then although the problem of volatilization of the surface treatment agent over time can be suppressed, because the molecular weight of the treatment agent is comparatively large, the surface treatment action on the inorganic filler is inferior to that provided by an equal mass and equal volume of a silane-based treatment agent. As a result, a much larger quantity of the treatment agent is required to achieve a satisfactory effect. Accordingly, the development of a surface treatment agent for a heat-conductive filler that suffers minimal variation over time and yields a favorable effect with only a small quantity, a heat-conductive silicone composition that uses such a surface treatment agent, and a cured product of such a composition has been keenly sought.    [Patent Reference 1] JP 47-32400 A    [Patent Reference 2] EP 0 024 498 A1    [Patent Reference 3] EP 0 032 050 A2    [Patent Reference 4] JP 1-69661 A    [Patent Reference 5] U.S. Pat. No. 5,352,731    [Patent Reference 6] JP 3290127 B2    [Patent Reference 7] U.S. Pat. No. 6,306,957    [Patent Reference 8] US 2006/0135687 A1